Mitochondrial are complex organelles whose cellular roles extend beyond the well-characterized metabolic pathways of respiration, fatty-and amino-acid metabolism and ion homeostasis. Their central role in the cell is reflected by the fact that they function as regulators of apoptosis. Mitochondria are also dynamic organelles, which continually undergo fission and fusion. Using the yeast S. cerevisiae as our experimental system, we have identified proteins directly required for mitochondrial fission and fusion, whose orthologs in mammalian cells also regulate apoptosis. Among them are 3 dynamin-related proteins (DRPs), which are large self-assembling GTPases that regulate membrane dynamics in a variety of cellular processes. To date, our work has placed these key players into a framework of molecular events that occur during fission and fusion. We propose to build on these studies to understand in more detail the mechanism of how these proteins collaborate to carry out the mechanics of membrane fission and fusion. Towards this goal, we have established in vitro assays that recapitulate mitochondrial fusion and mitochondrial fission events. We will use these assays combined with genetic and cytological approaches to determine the molecular mechanism of mitochondrial fission and fusion. Mutations in 2 conserved human mitochondrial fusion proteins have been linked to the neurodegenerative diseases, Charcot-Marie-Tooth and dominant optic atrophy. Resolving the mechanisms of mitochondrial fission and fusion will provide insight into the etiology of these fusion protein-linked diseases and will help to understand how mitochondrial dynamics regulates apoptosis.